Effect of textures on oxygen reduction electrocatalysis by self-supporting Fe-N-C materials from a universal template synthesis

Self-supporting Fe-N-C materials with certain ordered mesopores were synthesized through confined carbonization, alkali etching and final anneal by utilizing mesoporous silicas with varied textural properties, polymerized aniline and iron chloride as the hard template, carbon/nitrogen and iron precursors, respectively. These duplicated Fe-N-C materials have orderly mesoporous structure with large pore diameter, pore volume and specific surface area. As a result, all of them showed excellent electrocatalytic activity towards oxygen reduction reaction (ORR) in 0.1 M KOH with 4-electron pathway and half-wave potential (E _1/2 ) > 0.82 V vs. RHE, indicating that the nano-casting method using mesoporous silicas as the hard template is a universal approach for constructing highly active Fe-N-C catalysts. The activity order of PANI-Fe-HT2(MCF) > PANI-Fe-HT2(MCM-48) ≈ PANI-Fe-HT2(KIT-6) > PANI-Fe-HT2(SBA-15) > PANI-Fe-HT2(MCM-41) suggests that the three dimensional (3D) interconnected mesoporous structure of hard templates (MCF, MCM-48 and KIT-6) is generally more favorable for fabricating high-performing Fe-N-C materials. The highest onset potential (E _onset ) and E _1/2 obtained on the PANI-Fe-HT2(MCF) reached 0.99 and 0.86 V, respectively, which even slightly surpass those of commercial Pt/C catalyst, probably shedding light on the additional contribution of the unique pore structure (large window diameter and spherical chamber) duplicated from the MCF to further enhancement of ORR activity. The PANI-Fe-HT2(MCF)-based zinc-air battery delivered impressing power density (116.26 mW cm ^-2 ) and specific capacity (788 mAh g _Zn ^-1 ) as well, completely rivaling the Pt/C-based ZAB. The universal method will hold great promise for exploiting efficient Fe-N-C materials for ORR towards their practical applications to device.